1. Field of the Invention
This invention relates generally to the field of controlling the rate of dissolution of solid chemical material, such as in tablets, granules, etc., into liquid solution. In particular the invention relates to controlling the rate of dissolving calcium hypochlorite tablets or "pucks" in water for making the water safe for drinking by human beings.
2. Description of the Prior Art
Water used for human consumption, food processing, industrial cooling, washing, swimming and lubrication requires various levels of disinfection in order to prevent growth of various bacteria and fungi that threaten human health as well as damage industrial equipment and processes. Calcium hypochlorite is but one of several popular chemicals used for disinfection. However, calcium hypochlorite must be introduced in a controlled manner which will ultimately produce residuals in water in the range of 0.1-5 PPM (parts per million).
Erosion has been the primary means of dissolving solid chemical tablets or granules in the past. Submitting chemical tablets, pellets, solid cylindrical sticks or granules to a controlled flow rate of water has been used to dissolve the solid. Varying the flow rate, which in effect changes the rate of erosion of the solid chemical material through surface friction of liquid abrading against the solid, has been the primary means of control. A higher flow rate dissolves more chemical material into the water, and vice versa.
Varying of the flow rate accomplishes one of two goals. It either increases velocity of the water against the same amount of solid chemical material, or raises the liquid level in the reservoir which increases the amount of wetted surface of the solid chemical material. A combination of a more intense erosive action or greater wetted surface area of the chemical material theoretically changes the concentration of the solution being created by the process.
Current erosion feeder technologies all share common shortcomings. Since the various shapes of solids are typically stacked either in an orderly form or applied randomly within a container, their surface geometry varies relative to the direction of flow of the dissolving fluid. Some surfaces are perpendicular to the flow and some are at various angles; therefore the erosive effect of the fluid against the solid varies throughout a given load of tablets. Since solid tablets are typically arranged in a vertical column above the wetted area, and are fed through gravity weight of the tablets as dissolution takes place, the dissolution rate also varies with any given flow rate of fluid across the exposed tablets. As the dissolution varies, so does the strength of the resulting solution exiting the system.
Erosion feeders break down solid chemical tablet structure largely through the action of water molecules physically abrading the solid chemical material (e.g. tablet), causing it to be dissolved. The erosion action of the water on the tablets of the feeder varies with changes in velocity and resulting intensity of the eroding fluid striking the tablet. Therefore, subtle differences in the shape and attitude of the chemical solid material in relation to the direction of flow of fluid affects the rate and consistency of erosion. Erosion rates are further influenced by increasing or decreasing the amount of solid chemical material being exposed to the eroding flow of fluid. This occurs when the fluid level within the feeder is changed or when the fluid flow rate within the feeder is changed.
Controlling the flow rate of the dissolving fluid across the wetted area of the solid chemical material requires various controls, valves and fluid flow measuring equipment. Since system supply pressure of such dissolving fluid can and often does vary, maintaining a consistent erosion rate and subsequent solution strength is extremely difficult, often requiring a complex system of controls. If automatic controls are called for, the mechanical actuation of such controls is often complex and costly.
"Turn-down", or the ability to vary the amount of chemical into liquid of the feeder, is extremely important. Since erosion rates of various solid chemical materials (of tablets or granules) depends largely on a physical action point, the rate of erosion often becomes unpredictable in the lower ranges. Current technology erosion feeders do not typically produce a linear response to changes in flow rates. As a result, when systems are "turned-down" from very high chemical dissolution levels to very low levels, consistency and accuracy is sacrificed. Therefore, large systems are not generally capable of delivering very small chemical dissolution levels, and smaller systems have upper limits due to volumetric capacity of both solid chemical and eroding fluid.
Erosion feeders depend on relatively large volumes of liquid to effect the erosion process. Therefore, applications requiring small amounts of dissolved chemical at a very precise rate are not generally applicable. Fluid handling equipment such as pumps and piping must be sized to handle large volumes of liquid, and since the control of the system is accomplished through varying the flow rate through the chemical deliverer (e.g., chlorinator), the equipment used to deliver the final solution must also be controllable, making the choice and arrangement of these various flow controlling devices critical to feeder performance.
Erosion is further complicated because the process of producing various chemicals in solid form depends on the application of chemical binders and hydraulic compaction. Even subtle variations in the manufacturing process causes inconsistencies in dissolution rates, because the solids often have soft or hard areas within the solid form. This factor makes water velocity and angle of incidence even more critical in the erosion process.
Since many water disinfection applications require extremely small amounts of chemical residual to be placed into solution, current technology does not provide consistent performance due to excessive flow rates required to effect dissolution. The inconsistent performance has been due to difficulties in controlling the flow volume and complexity of the system. As a result prior technologies have not been cost-effective. Installations such as very low volume water wells and chemical processes require only minute quantities of chemical in very small amounts of treating solution. Current technology cannot provide these levels of delivery and consistency since residual chemical must be delivered in as little as 0.5 parts per million.